1. Introduction

I've got a
laptop computer running Linux and I don't want to
worry about someone reading the personal information it contains, in case
it gets lost or stolen. My log on password may stop someone from booting
it, but it won't prevent an attacker from removing the hard disk and
reading its data. I need stronger protection.

Fortunately, it's relatively easy to use encryption so the hard
disk data would be unreadable if it were to fall into the wrong hands.
Encryption's not only useful for portable computers like laptops—it
can be used to protect any computer with personal information. I protect
my computer's files with encryption for the same reason I lock my
filing cabinet at home. For further motivation, you may be interested in
reading Michael Crawford's Why You Should Use Encryption.

I could encrypt only certain files, such as those in my home
directory. This would protect the files but then I'd have to worry
about information leaking out of them to other, unencrypted places on the
disk. Instead I encrypt the whole disk so I don't have to manage this
problem.

There are many encryption algorithms to choose from. I chose AES because it has
been approved by the US government's National
Institute of Standards and Technology and is well regarded by the
cryptography community. I want my use of it to be resistant to dictionary
attacks, so I use a long, randomly generated key. There's no way
I'm going to memorize such a key so I keep it in a form I can carry
with me easily: on a USB flash drive on my keychain. I encrypt the key
with a passphrase so my data is protected in two ways: by a) what I have
(the USB flash drive) and b) what I know (the passphrase). I can even give
a friend access to my computer without giving away my passphrase—she uses
her own USB flash drive and her own passphrase.

The operating system keeps the data encrypted on the disk at all
times and decrypts it in RAM only as it's used. This way if the
computer loses power suddenly the data will remain protected. The
decryption key is loaded into RAM at boot time and kept there while the
computer is on, so I don't need to keep the USB flash drive plugged in
after starting the computer.

The procedure outlined in this HOWTO is written for version 2.4 of
the Linux kernel. It will become less complicated with the release of
Linux 2.6, which will have built-in support for encryption and do a better
job of managing partitions within loopback devices.

1.1. Technical Summary

The encryption is implemented through a special kind of
loopback device. A loopback device doesn't
store any data itself; instead it takes all the data storage and
retrieval requests it receives and passes them along to a real storage
device, such as a disk or a file. As the data passes through, it can be
filtered, and in our case the filter used is encryption.

When the system is deployed, a removable medium (USB flash drive)
boots using GRUB, a kernel, and an initrd. Both the key and the kernel
are selected from the GRUB menu, allowing a single removable medium to
be used with multiple computers. The initrd contains just enough tools
to ask for a passphrase, set up an encrypted loopback device, and mount
it. After mounting, pivot_root is used to resume the
boot process from the encrypted device. Loopback device offsets are
used, instead of partitions, to access separate swap and root file
system spaces within the encrypted loopback device because the 2.4
kernel doesn't provide access to partitions within loopback devices.
The offset method does not generalize to multiple partitions
(unfortunately) because the maximum offset understood by
losetup is 2GB.

1.2. Copyright and License

Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License, Version
1.2 or any later version published by the Free Software Foundation; with
no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
copy of the license is included in Appendix A.

Linux is a registered trademark of
Linus Torvalds.®

1.3. Disclaimer

No liability for the contents of this document can be accepted.
Use the concepts, examples and information at your own risk. There may
be errors and inaccuracies that could be damaging to your system and you
may lose important data. Proceed with caution, and although this is
highly unlikely, the author does not take any responsibility.

All copyrights are held by their by their respective owners,
unless specifically noted otherwise. Use of a term in this document
should not be regarded as affecting the validity of any trademark or
service mark. Naming of particular products or brands should not be seen
as endorsements.

I know you hate reading directions and want to skip to the meaty
bit right away, but I advise you to read the whole document first before
touching anything. I know all the HOWTOs say that, but I really mean it
for this one. It's worth it; trust me. You may also want to run
through the procedure first on a test system before tackling a
production system.

1.4. Acknowledgments

Thanks to Linus Torvalds, Jari Ruusu, and all the developers who
contributed to their software, without which this HOWTO would have been
impossible.

Thanks to alert readers Ladislao Bastetti and Norris Pouhovitch for
struggling through unusual hardware configurations, finding mistakes in
the HOWTO, and suggesting good ideas.

1.5. Feedback

Feedback is solicited for this document. Please send additions,
comments, and criticisms to the author.

1.6. Approaches

There are three different approaches we can take to encrypt the
disk: encrypt the whole thing, a single partition, or a single file. I
strongly recommend the first approach for best security. The first two
approaches assume you'll be booting from removable media, such as a
USB flash drive or a business card size CD-ROM. If you don't want to
do this, you may modify the method to boot from the disk instead by
making a small, unencrypted boot partition. If you want to use a USB
flash drive to boot your computer, be sure your motherboard can do it
first. At the time of this writing many cannot.

To avoid having to enumerate all three approaches everywhere
I'm going to refer to what you're protecting as the
asset. I will refer to the removable medium used
to store the key as the keychain. I call it the
keychain instead of the key because we can store lots of keys, each for
different computers, on the same medium.

1.6.1. Whole Disk

A problem with keeping data secret with encryption is that the
data likes to move around. Imagine the encryption is like a fence
around your data. While the data's inside the fence, it's
safe. To be most useful, however, data likes to be transmitted on
networks, put on removable disks like CD-ROMs, and shared with
friends. Any time your data leaves the fenced area it's
unprotected. We can't put an encryption fence around all possible
locations where our data might play but we do want to make the fence
as large as practical. By putting the encryption fence around your
whole hard disk, you won't have to worry about data becoming
unprotected if it jumps to another part of the disk.

In this approach, we create one swap
space and one root file system. Some people want more than a single
encrypted partition for the root file system. Unfortunately, the
method detailed here relies on the offset parameter of
losetup to create "subpartitions" within
the asset. The offset parameter is limited to a maximum value of 2GB,
limiting the size of all but the last partition to 2GB. This works
nicely for swap, which is already limited to 2GB on the i386
architecture, but I'm guessing it won't be practical for other
uses. Using it to create multiple partitions smaller than 2GB is left
as an exercise for the reader.

Another way to handle multiple partitions is to encrypt each partition separately (using the same key) to avoid the technical limitation above. This isn't secure as encrypting the whole disk because the partition table is exposed. When an attacker attempts to break encryption, the first thing he does is try to figure out what it's encrypting. A partition table listing Linux partitions is a big hint. For this reason I discourage encrypting multiple partitions separately, but arguably it's a good compromise for getting around the current losetup limitation. Another option is simply to wait for the release of Linux 2.6 because it is expected to make the offset parameter unnecessary.

1.6.2. Partition (for multiboot systems)

Encrypting the whole disk is fine if Linux is the only operating
system on it, but this won't work for people who have set up their
computer to boot multiple operating systems, e.g., Linux, NetBSD, and Darwin. In this case
we can encrypt just the Linux partition and leave the others alone.
Since we're booting from a removable medium, we won't even
need to include the Linux partition in the multiboot menu with the
others. To see why this isn't as secure as encrypting the whole
disk, read Table 1.

1.6.3. File (for home directories)

You may want to encrypt only a file on a file system. Once
you've encrypted it you can put into it whatever you want,
including other file systems. You might want to use this approach to
encrypt only your home directory, for example. This is the least
secure of the three approaches and not recommended. If you choose this
approach you will notice instructions below to skip whole sections.
This is because I'm assuming you've already booted an
operating system and have your swap issues handled, so those sections
don't apply to you. This HOWTO may be overkill for your needs and
you can probably get away with just reading the fine
README
that comes with loop-AES.
If you do, be sure to read Section 1.7 before you
finish here.

1.7. Threat Model

In order to protect our asset well, we must first understand what
we're protecting it against. The general idea is that you've got
a laptop which is vulnerable to being stolen or lost, and have a USB
flash drive on your keychain that isn't, so this system is designed
to handle the case that your laptop is stolen. I'm guessing your
keychain won't be as easily stolen because it's in your pocket,
and because an attacker won't know that it's important. If you
pull your USB flash drive out of your pocket and someone non-technical
exclaims, "What's that?", tell them it's a Pez dispenser.

This system falls short when it comes to plausible
deniability, which means there's no way to hide the
fact that your personal data is encrypted. This is like locking your
jewels in a safe and keeping the safe in plain sight in the middle of
your living room. Only you can open the safe, but a man with a gun can
tell you to open the safe for him. So if you're worried about your
computer
being subpoenaed and being told to hand over your laptop,
keychain, and passphrase, you'd better look at other solutions
such as StegFS.

The following solution to the deniability problem has been suggested by Norris Pouhovitch.
It should be possible to install a minimal Windows partition at the front
of the disk and to encrypt the remainder. When the computer
is turned on without the keychain, it boots Windows normally. When the
keychain is booted, it skips the Windows partition, decrypts the remainder of
the disk, and boots Linux.

The advantage of this scheme is that if the laptop is stolen and turned
on, it will look like what a casual attacker is expecting to see (a Windows
computer). On the other hand, a serious attacker could notice the unusually
small partition and become suspicious. I will flesh out this idea further in
a future version of the HOWTO.

The asset is unprotected while the computer is running because the encryption key is in RAM. You can lower the risk by using an idle logout (Section 2.6.3), but if you think your laptop is about to be stolen, turn off the power immediately and quickly read the Aikido HOWTO.

1.9.1. A Digression about USB Flash Drives

There are many choices on the market. When I bought mine, I
found one which fit the following requirements:

physically small (I carry it on my physical keychain)

supports USB 2.0 at full speed

has a write-protect switch, so I don't clobber my
encryption keys by accident

You might be tempted to get one with a fingerprint reader. I
strongly encourage you not to. It might initially seem like a good
idea, because by adding the biometric, your security protection
expands to:

something you have (the USB flash drive)

something you know (the passphrase)

something you are (your fingerprint, or whatever)

However, suppose something goes wrong. If you are now asking
yourself, "What could go wrong?", then why are you reading
this HOWTO? If something goes wrong, you make a change (see Corrective Reactions):

Change what you have by using a different USB flash drive.

Change what you know by learning a new passphrase.

You can't change what you are.

Stop and ponder that last line for a while.

1.10. Looking to the Future

I wrote this document while using the 2.4 kernel. Linux 2.6 introduces the Device-mapper which we will be able to use to avoid playing games with
losetup offsets. Linux 2.6 also introduces dm-crypt, an
encryption layer for the Device-mapper which looks quite elegant. Unfortunately, it's not safe! Hopefully someday
it will be fixed, but in the mean time the best course is to stick with loop-AES.

A future version of this HOWTO will explain how to use the Device-mapper with Linux 2.6.

2. Procedure

This method is designed to erase the contents of the asset before
encrypting it. If you already have data on the disk you intend to encrypt,
you should copy it somewhere else temporarily and then move it back once
the encryption is set up. It is possible to encrypt data in place, but for
now I consider such magic too advanced for this HOWTO. See loop-AES's
README
for more details if you're interested in that method.

To do the following operations you will need to be running a system
which has a loop-AES
capable kernel. If you don't have one already, I recommend using
KNOPPIX. It boots off a
CD-ROM and doesn't need to be installed, so it's very little
hassle.

For simplicity these instructions assume you'll be preparing the
keychain and the asset on the same computer, but this needn't be the
case. Adapt the instructions to whatever's convenient for you.

2.1. Prepare the Keychain

If you're taking the approach of encrypting only a file
instead of a disk or a partition, you may skip this section and proceed
directly to Section 2.2.

In the ideal setup you will use a bootable keychain device, such
as a USB flash drive or a business card size
CD-ROM. This is because we want to expose as little of your disk as
possible, but we're going to have to expose a minimal boot process
or the computer will never start. Since the boot process will be
necessarily unencrypted, it's better to have it away from your
computer (on your keychain). If you can't or don't want to use a
bootable keychain for some reason, then follow these instructions anyway
but instead apply them to a small boot partition on your disk instead of
the keychain.

In the following example the keychain shows up as the first SCSI
drive /dev/sda. Replace /dev/sda
with the device for your drive as appropriate.

The first step—zeroing out the keychain—is technically
unnecessary, but it will make the keychain backup smaller if you back
it up as an image as I suggest in Section 2.4.

bash# dd if=/dev/zero of=/dev/sda

Next, partition the keychain as you would any bootable
disk. See the Linux Partition
HOWTO if you need help with partitioning.

2.1.1. Build the Kernel

If you use the keychain with multiple computers you may
want to build a different kernel for each one.

You probably need to build a custom kernel for your keychain so
you can ensure two things:

It has been patched correctly with loop-AES
and encryption support is turned on.

All the device drivers necessary to boot your computer and
make the asset accessible have been compiled in instead of loaded
as modules.

You can load device drivers as modules, since we're using an
initrd, but I chose to compile them into the kernel in order to keep
the boot disk as simple as possible. Feel free to do differently.

For help building a custom kernel read The Linux
Kernel HOWTO. Be sure to set CONFIG_BLK_DEV_RAM
in the kernel configuration so it can boot using an initrd.

Follow the directions that come with loop-AES to
build the new loop driver. Also follow the directions to rebuild
the util-linux
tools, some of which we'll copy to the keychain later. Your distribution may have
already built them for you (e.g., see the loop-aes-utils and loop-aes-source packages in Debian).

2.1.2. Make the initrd

We boot the keychain using an initrd so we can remove it after
the boot process starts (who wants a USB flash drive hanging out of
their laptop while trying to look cool in a café?). To gain access to
the asset we create a loopback device attached to the initrd's
/dev/loop0. Putting the device file on the initrd
means the initrd will have to stay mounted while the asset is mounted (not a big deal).

2.2. Prepare the Asset

It's possible to repeat these steps as many times as you want
to handle multiple computers using the same keychain. Each computer will
have its own key and probably its own kernel. The instructions here
assume the computer's name is laptop;
substitute the name of the computer you're working with each time
you repeat the steps.

No, stop, listen to me. Back up your data. Really. It's no fun
to have an encrypted hard disk if you can't decrypt it because of
some mistake you made. These tools are powerful magic; if you blow it
you can't just call up Computer Gurus Are Us
and expect them to get your data back for you. That's the whole
point of this exercise.

If you are encrypting your whole disk (recommended), replace
/dev/hda with the device for your disk.

bash# ln -s /dev/hda /tmp/asset

If you are encrypting a partition (multiboot case), replace
/dev/hda3 with the device for your partition.

bash# ln -s /dev/hda3 /tmp/asset

If you are encrypting a file only, replace ~/encrypted
with the name of the file and create a link named /tmp/keychain
that points to where you decide to store your key file (an already
prepared removable medium, e.g., /mnt/cf).

bash# ln -s ~/encrypted /tmp/assetbash# ln -s /mnt/cf /tmp/keychain

Initialize the asset with random data. This will make it less
obvious to the attacker which parts are free space.

bash# shred -n 1 -v /tmp/asset

Here we create an encrypted file system to hold the keys. More
encryption, you say? Yes, in case your keychain is stolen (see Table 1), you don't want your keys to be exposed. I
chose one megabyte as the size of the file system because it's a
round number. There's no way we're going to need that much space
for keys so feel free to chose a smaller size if you like (each key file
will be 61 bytes long).

Again, initialize with random data.

bash# cd /tmp/initrdbash# head -c 1m /dev/urandom > keys

To make the passphrase resistant to dictionary attacks we'll
generate a seed. Whenever you see the symbol <seed>
be sure to replace it with the one you generated. The following command
will display a random seed on the screen.

bash# head -c 15 /dev/random | uuencode -m - | head -2 | tail -1

Set up the loopback device using the seed. This is where you
choose your passphrase, which must be at least 20 characters in length. Choose one with care that you know you won't forget. You may want to use the Diceware method for choosing a secure passphrase.

bash# losetup -e AES128 -C 100 -S <seed> -T /dev/loop1 keys

Format and mount the keys file system (the decrypt.sh script assumes you use the ext2
file system here).

Now for the actual asset key, 45 bytes as random as your computer
can make them. Try a dictionary attack against that, attacker! Ha! We
name the key after the computer with which it will be used (laptop). Substitute the name of your
computer instead.

2.2.2. Root File System

If you're encrypting only a file, format it with a file
system like this and skip to Section 2.3.

bash# mkfs /dev/loop0

We'll create the root "partition" after the swap
space. I put the word 'partition' in quotes because it's
not a real partition. We're faking it using the offset argument of
losetup.

Notice how mkswap told us the actual size of
the swapspace, which is not necessarily the size requested. Use the
actual size (which was 2147471360 in the above example) when
specifying the offset to begin the root file system.

bash# losetup -o <root offset> /dev/loop1 /dev/loop0

If the asset is the whole disk or the last partition on the
disk, then we needn't worry about specifying a size for the file
system. If this applies to you, do the following and skip to Section 2.2.2.1.

bash# mkfs /dev/loop1

Since the asset isn't the last partition on the disk, we
must give mkfs a size limitation or it will write
all over whatever partitions are between this one and the end of the
disk. I repeat, if you don't give mkfs
the correct size parameter here, you may lose data.
mkfs is actually just a front end, so to be as
careful as possible we'll choose an actual file system maker, in
this case mke2fs.

It's possible to limit the size of the file system by
specifying its size in blocks, but mke2fs chooses
the block size based on the size of the file system. A classic
Catch-22! We can ask it to do a dry run on the rest of the disk (more
than we want) to see what block size it would chose.

2.4. Testing and Backup

Test your system by booting the keychain or executing the decrypt.sh script as appropriate (give it the name of the
key you want to use as a parameter). After booting there may be a
complaint about a nonexistent /sbin/init but
that's okay for now.

Check to make sure your root file system mounted successfully.
When you're confident everything is working, back up your keychain.
In fact, make lots of backups. You might ask, "But isn't it
insecure to have a copy of my keychain somewhere?" The answer is
yes, it is, but not as insecure as losing your only keychain, if you
define security as also meaning "securing access to my data".

Because my keychain is small I decided to back up the whole image
so it's easy to restore:

bash# bzip2 -c /dev/sda > keychain.img.bz2

If you're encrypting only a file, you can pat yourself on your
back at this point because you've finished.

2.5. Rescue Disk

Rescue disks are useful when a system isn't behaving properly
and/or refuses to boot. Check to make sure your rescue disk has loop-AES support
in its kernel and has the correctly patched util-linux
tools such as losetup and mount,
otherwise it will be worthless with your newly encrypted asset. In the
future, all rescue disks will include this support because it will come
standard with the 2.6 kernel. In the meantime, KNOPPIX (for example) already has
all the necessary support and can be used as a rescue disk.

2.6.3. Idle Logout

Once your system is up and running, consider configuring it to log out automatically after a period of inactivity. This will lessen (but not eliminate) the risk of exposing your asset if the laptop is stolen while on (see Table 1).

3. More Information

The README
that comes with loop-AES
explains how to use it in multiple scenarios.

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give them a chance to provide you with an updated version of the
Document.

A.5. MODIFICATIONS

You may copy and distribute a Modified Version of the Document
under the conditions of sections 2 and 3 above, provided that you
release the Modified Version under precisely this License, with the
Modified Version filling the role of the Document, thus licensing
distribution and modification of the Modified Version to whoever
possesses a copy of it. In addition, you must do these things in the
Modified Version:

Use in the Title Page (and on the covers, if any) a title
distinct from that of the Document, and from those of previous
versions (which should, if there were any, be listed in the History
section of the Document). You may use the same title as a previous
version if the original publisher of that version gives permission.

List on the Title Page, as authors, one or more persons or
entities responsible for authorship of the modifications in the
Modified Version, together with at least five of the principal
authors of the Document (all of its principal authors, if it has
fewer than five), unless they release you from this requirement.

State on the Title page the name of the publisher of the
Modified Version, as the publisher.

Preserve all the copyright notices of the Document.

Add an appropriate copyright notice for your modifications
adjacent to the other copyright notices.

Include, immediately after the copyright notices, a license
notice giving the public permission to use the Modified Version
under the terms of this License, in the form shown in the Addendum below.

Preserve in that license notice the full lists of Invariant
Sections and required Cover Texts given in the Document's
license notice.

Include an unaltered copy of this License.

Preserve the section Entitled "History", Preserve
its Title, and add to it an item stating at least the title, year,
new authors, and publisher of the Modified Version as given on the
Title Page. If there is no section Entitled "History" in the
Document, create one stating the title, year, authors, and publisher
of the Document as given on its Title Page, then add an item
describing the Modified Version as stated in the previous sentence.

Preserve the network location, if any, given in the
Document for public access to a Transparent copy of the Document,
and likewise the network locations given in the Document for
previous versions it was based on. These may be placed in the
"History" section. You may omit a network location for a
work that was published at least four years before the Document
itself, or if the original publisher of the version it refers to
gives permission.

For any section Entitled "Acknowledgements" or
"Dedications", Preserve the Title of the section, and
preserve in the section all the substance and tone of each of the
contributor acknowledgements and/or dedications given therein.

Preserve all the Invariant Sections of the Document,
unaltered in their text and in their titles. Section numbers or the
equivalent are not considered part of the section titles.

Delete any section Entitled "Endorsements". Such a
section may not be included in the Modified Version.

Do not retitle any existing section to be Entitled
"Endorsements" or to conflict in title with any Invariant
Section.

Preserve any Warranty Disclaimers.

If the Modified Version includes new front-matter sections or
appendices that qualify as Secondary Sections and contain no material
copied from the Document, you may at your option designate some or all
of these sections as invariant. To do this, add their titles to the list
of Invariant Sections in the Modified Version's license notice.
These titles must be distinct from any other section titles.

You may add a section Entitled "Endorsements", provided it
contains nothing but endorsements of your Modified Version by various
parties--for example, statements of peer review or that the text has
been approved by an organization as the authoritative definition of a
standard.

You may add a passage of up to five words as a Front-Cover Text,
and a passage of up to 25 words as a Back-Cover Text, to the end of the
list of Cover Texts in the Modified Version. Only one passage of
Front-Cover Text and one of Back-Cover Text may be added by (or through
arrangements made by) any one entity. If the Document already includes a
cover text for the same cover, previously added by you or by arrangement
made by the same entity you are acting on behalf of, you may not add
another; but you may replace the old one, on explicit permission from
the previous publisher that added the old one.

The author(s) and publisher(s) of the Document do not by this
License give permission to use their names for publicity for or to
assert or imply endorsement of any Modified Version.

A.6. COMBINING DOCUMENTS

You may combine the Document with other documents released under
this License, under the terms defined in section
4 above for modified versions, provided that you include in the
combination all of the Invariant Sections of all of the original
documents, unmodified, and list them all as Invariant Sections of your
combined work in its license notice, and that you preserve all their
Warranty Disclaimers.

The combined work need only contain one copy of this License, and
multiple identical Invariant Sections may be replaced with a single
copy. If there are multiple Invariant Sections with the same name but
different contents, make the title of each such section unique by adding
at the end of it, in parentheses, the name of the original author or
publisher of that section if known, or else a unique number. Make the
same adjustment to the section titles in the list of Invariant Sections
in the license notice of the combined work.

In the combination, you must combine any sections Entitled
"History" in the various original documents, forming one section
Entitled "History"; likewise combine any sections Entitled
"Acknowledgements", and any sections Entitled
"Dedications". You must delete all sections Entitled
"Endorsements".

A.7. COLLECTIONS OF DOCUMENTS

You may make a collection consisting of the Document and other
documents released under this License, and replace the individual copies
of this License in the various documents with a single copy that is
included in the collection, provided that you follow the rules of this
License for verbatim copying of each of the documents in all other
respects.

You may extract a single document from such a collection, and
distribute it individually under this License, provided you insert a
copy of this License into the extracted document, and follow this
License in all other respects regarding verbatim copying of that
document.

A.8. AGGREGATION WITH INDEPENDENT WORKS

A compilation of the Document or its derivatives with other
separate and independent documents or works, in or on a volume of a
storage or distribution medium, is called an "aggregate" if the
copyright resulting from the compilation is not used to limit the legal
rights of the compilation's users beyond what the individual works
permit. When the Document is included in an aggregate, this License does
not apply to the other works in the aggregate which are not themselves
derivative works of the Document.

If the Cover Text requirement of section 3 is applicable to these
copies of the Document, then if the Document is less than one half of
the entire aggregate, the Document's Cover Texts may be placed on
covers that bracket the Document within the aggregate, or the electronic
equivalent of covers if the Document is in electronic form. Otherwise
they must appear on printed covers that bracket the whole aggregate.

A.9. TRANSLATION

Translation is considered a kind of modification, so you may
distribute translations of the Document under the terms of section 4.
Replacing Invariant Sections with translations requires special
permission from their copyright holders, but you may include
translations of some or all Invariant Sections in addition to the
original versions of these Invariant Sections. You may include a
translation of this License, and all the license notices in the
Document, and any Warranty Disclaimers, provided that you also include
the original English version of this License and the original versions
of those notices and disclaimers. In case of a disagreement between the
translation and the original version of this License or a notice or
disclaimer, the original version will prevail.

If a section in the Document is Entitled
"Acknowledgements", "Dedications", or "History",
the requirement (section 4) to Preserve its Title (section 1) will
typically require changing the actual title.

A.10. TERMINATION

You may not copy, modify, sublicense, or distribute the Document
except as expressly provided for under this License. Any other attempt
to copy, modify, sublicense or distribute the Document is void, and will
automatically terminate your rights under this License. However, parties
who have received copies, or rights, from you under this License will
not have their licenses terminated so long as such parties remain in
full compliance.

A.11. FUTURE REVISIONS OF THIS LICENSE

The Free Software Foundation may publish new, revised versions of
the GNU Free Documentation License from time to time. Such new versions
will be similar in spirit to the present version, but may differ in
detail to address new problems or concerns. See
http://www.gnu.org/copyleft/.

Each version of the License is given a distinguishing version
number. If the Document specifies that a particular numbered version of
this License "or any later version" applies to it, you have the
option of following the terms and conditions either of that specified
version or of any later version that has been published (not as a draft)
by the Free Software Foundation. If the Document does not specify a
version number of this License, you may choose any version ever
published (not as a draft) by the Free Software Foundation.

A.12. ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy
of the License in the document and put the following copyright and
license notices just after the title page:

Copyright (c) YEAR YOUR NAME. Permission is granted to copy,
distribute and/or modify this document under the terms of the GNU Free
Documentation License, Version 1.2 or any later version published by
the Free Software Foundation; with no Invariant Sections, no
Front-Cover Texts, and no Back-Cover Texts. A copy of the license is
included in the section entitled "GNU Free Documentation
License".

If you have Invariant Sections, Front-Cover Texts and Back-Cover
Texts, replace the "with...Texts." line with this:

with the Invariant Sections being LIST THEIR TITLES, with the
Front-Cover Texts being LIST, and with the Back-Cover Texts being
LIST.

If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.

If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of free
software license, such as the GNU General Public License, to permit
their use in free software.